Bash - Check if a Variable is Set - Tutorial Kart

Bash ¨C Check if variable is set

Bash ¨C Check if variable is set

To check if a variable is set in Bash Scripting, use-v var or-z ${var} as an expression with if command.

This checking of whether a variable is already set or not, is helpful when you have multiple script files, and the

functionality of a script file depends on the variables set in the previously run scripts, etc.

In this tutorial, we shall learn the syntax and usage of the above mentioned expressions with examples.

Syntax

Following is the syntax of boolean expressions which check if the variable is set:

[[ -v variableName ]]

[[ -z variableName ]]

The boolean expression returns true if the variable is set and false if the variable is not set.

Example 1 ¨C Check if Variable is Set using -v

In this example, we use [[ -v variableName ]] boolean expression to check if variables a and b

are set with the help of bash if else statement.

Bash Script File

#!/bin/bash

a=10

# a: variable is set

if [[ -v a ]];

then

echo "variable named a is already set"

else

echo "variable a is not set"

fi

fi

# b: variable is not set

if [[ -v b ]];

then

echo "variable named b is already set"

else

echo "variable b is not set"

fi

Output

~/workspace/bash$ ./bash-if-variable-is-set-example

variable named a is already set

variable b is not set

Variable a is defined and assigned a value of 10 and hence is set. For variable b , we have not defined it by

assigning a value. So, we got the result that variable b is not set.

Example 2 ¨C Check if Variable is Set using -z

In this example, we use [[ -z ${variableName} ]] boolean expression to check if variables a and

b are set with the help of bash if else statement.

Bash Script File

#!/bin/bash

a=10

# a: variable is set

if [[ -z ${a} ]];

then

echo "variable a is not set"

else

echo "variable named a is already set"

fi

# b: variable is not set

if [[ -z ${b} ]];

then

echo "variable b is not set"

else

echo "variable named b is already set"

fi

Output

~/workspace/bash$ ./bash-if-variable-is-set-example

variable named a is already set

variable b is not set

Conclusion

In this Bash Tutorial, we have learnt to check if a variable is set or not using [[ -v variableName ]] or [[ -z

${variableName} ]], with the help of example Bash scripts.

Bash Shell Scripting

? Bash Tutorial

? Bash Script Example

? Bash File Extension

? Bash Echo

? Bash Comments

? Bash Variable

? Bash Command Line Arguments

? Bash Read User Input

? Bash Read Password

? Bash Date Format

? Bash Sleep

Operators

? Bash Arithmetic Operators

Conditional Statements

? Bash If

? Bash If Else

? Bash Else If

? Bash Case

Loops

? Bash For Loop

? Bash While Loop

? Bash Until Loop

Strings

? Bash String Manipulation Examples

? Bash String Length

? Bash If String Equals

? Bash Split String

? Bash SubString

? Bash Concatenate String

? Bash Concatenate Variables to Strings

Functions

? Bash Function

? Bash Override Buitlin Commands

Arrays

? Bash Array

Files

? Bash Write to File

? Bash Read File

? Bash Read File line by line

? Bash If File Exists

? Bash If File is Directory

? Bash If File is Readable

Bash Others

? Bash Check if variable is set

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